1. Field of the Invention
The present invention is related to a fuel injection valve drive control apparatus, and particularly to a fuel injection valve drive control apparatus which controls an driving of the electromagnetic valve used in the fuel injection system of an internal combustion engine.
2. Description of the Prior Art
Generally, in an electromagnetic fuel injection valve system, as shown in FIG. 3, a plunger 2 is urged into abutment with a tapered fuel injection port 1 by a spring 3 thereby to maintain the value in a closed state. When a current is applied to a coil L surrounding the plunger 2, the plunger 2 is attracted and moves in the direction of an arrow "a" against the spring force of the spring 3 to open the valve. As a result, fuel is injected through the gap between the plunger 2 and the fuel injection port 1. Thus, in the prior art fuel injection valve, since the injection valve opens only for the period of time of a pulse current provided to the coil L, control of the fuel injection amount is performed by controlling the width of the pulse current.
In such a construction, even if the pulse current is supplied to the coil L at the time of valve opening, the valve does not open until a force overcoming the spring force acts on the plunger 2, and hence a time delay will occur. In addition, even if the fuel injection pulse turns off at the time of valve closing, the plunger 2 does not promptly return because of the residual magnetic flux in the plunger 2. Accordingly, such a fuel injection valve inherently has the problem that it is difficult to accurately control the injection amount in response to a fuel injection pulse used for opening the value.
To deal with such problem, it has been proposed, as shown in FIG. 4, that during the ON duration of the fuel injection pulse, a relatively large excitation current (valve open current) is caused to flow at the initial stage of valve opening to achieve a prompt valve opening operation, and once the valve opens, only a minimum excitation current (holding current) required for keeping the valve open is supplied to reduce the residual magnetic flux present when the coil current decreases to close the value. Further, to efficiently absorb the energy stored in the coil of the electromagnetic valve when the holding current is shut off, an apparatus provided with a so-called flywheel circuit is proposed, for instance, in the Japanese Patent Laid-open Nos. 51-125932 and 57-203830 official gazettes.
FIG. 5 is a circuit diagram showing the main portions of a fuel injection valve drive control apparatus including a flywheel circuit, and FIG. 6 is a waveform diagram of the driving signals thereof. One end of an electromagnetic coil L is connected to the emitter of a transistor Q.sub.1, and a battery voltage V.sub.B is applied to the collector of the transistor Q.sub.1. The other end of the coil L is grounded through a resistor R. In parallel with the coil L and the resistor R, a transistor Q.sub.2 and a diode D constituting the flywheel circuit are connected in series.
When a pulse signal (c) of FIG. 6 for chopping control is input to the base of the transistor Q.sub.1 in response to an fuel injection pulse (a), the transistor Q.sub.1 turns on, and an excitation current I.sub.L begins to flow through the coil L and gradually increases with a first-order time-lag as shown in (b) of the same figure.
When the excitation current I.sub.L reaches a valve-opening current I.sub.1 necessary for opening the closed electromagnetic valve and attraction of the plunger 2 is completed, the control pulse (c) falls down to an "L"-level, the transistor Q.sub.1 turns off, and the current I.sub.L of the coil L begins to decrease. When the excitation current I.sub.L falls to a lower limit value I.sub.2 of the holding current, the transistor Q.sub.1 again turns on and the excitation current I.sub.L starts to flow, and when the excitation current I.sub.L reaches the upper limit value I.sub.3 of the holding current, the transistor Q.sub.1 again turns off. Thereafter, such intermittent control of the transistor Q.sub.1 is repeated while the fuel injection pulse (a) is at a "H"-level, whereby the excitation current I.sub.L is maintained at a minimum current value (holding current) required for attracting and holding the plunger 2.
Since the transistor Q.sub.2 is controlled so that it turns on simultaneously with the leading edge of the injection pulse (a) as in FIG. 6(d), or simultaneously with the first turn-off of the transistor Q.sub.1 as in (e) of FIG. 6, the energy stored in the coil L is absorbed in the flywheel diode D each time the transistor Q.sub.1 is turned off.
This prior art apparatus had a problem that the base current should be continuously supplied to the transistor Q.sub.2 to activate the flywheel circuit for a relatively long time, resulting in large power consumption.